Copper is an essential nutrient. Despite the importance of copper to human nutrition, little is about the mechanisms regulating copper homeostasis in mammalian cells. Menkes disease is the only known genetic copper deficiency disorder and is caused by mutations in the Menkes gene (ATP7A; MNK). The classical form of Menkes disease is lethal in early childhood and caused by defective absorption and distribution of dietary copper within the body. There are milder allelic variants of the disease, which differ in phenotype and survival potential. The Menkes gene product (MNK) is a copper transporting P-type ATPase involved in the efflux of copper from cells. This function of the MNK protein is defective in Menkes disease patients, resulting in the entrapment of copper in the intestinal mucosa and a systemic copper deficiency. The MNK protein is expressed ubiquitiously in non-hepatic tissues, and is thought to be essential in copper transport across the blood brain barrier to the brain. MNK is located in the final compartment of the Golgi apparatus known as the trans-Golgi network (TGN). The MNK protein constitutively cycles in vesicles between the TGN and the plasma membrane under basal copper conditions. The exocytic trafficking of MNK is increased when copper levels become elevated, resulting in a redistribution of MNK to the plasma membrane where the protein effluxes copper to restore copper homeostasis. Our preliminary results suggest MNK is also essential for copper loading cuproenzymes in secretory compartments. Hence, the need for copper efflux by MNK and copper transport cuproenzymes is balanced by the cycling of the MNK between the TGN and plasma membrane. These functions of MNK are likely to underlie its importance in mediating copper entry into the body, copper transport to the brain, the activation of secreted cuproenzymes and the maintenance of copper homeostasis in most non-hepatic cells. Our long-term goal is to understand how the MNK protein functions in copper homeostasis and the molecular basis of Menkes disease. To achieve this goal we propose the following specific aims, 1) Characterize the effects of phenotypically diverse Menkes disease mutations on the function of the MNK protein; 2) Investigate the function of the Menkes protein in a genetic model of copper deficiency; and 3) Determine whether the copper-induced trafficking of MNK is dependent on the formation of a phosphorylated catalytic intermediates. These studies will provide great insight into the subcellular mechanisms regulating copper homeostasis.